gh-118362: Fix thread safety around lookups from the type cache in the face of concurrent mutators (#118454)

Add _PyType_LookupRef and use incref before setting attribute on type
Makes setting an attribute on a class and signaling type modified atomic
Avoid adding re-entrancy exposing the type cache in an inconsistent state by decrefing after type is updated

Lib/test/test_class.py

@@ -882,6 +882,25 @@ class TestInlineValues(unittest.TestCase):
         f.a = 3
         self.assertEqual(f.a, 3)
 
+    def test_store_attr_type_cache(self):
+        """Verifies that the type cache doesn't provide a value which  is
+        inconsistent from the dict."""
+        class X:
+            def __del__(inner_self):
+                v = C.a
+                self.assertEqual(v, C.__dict__['a'])
+
+        class C:
+            a = X()
+
+        # prime the cache
+        C.a
+        C.a
+
+        # destructor shouldn't be able to see inconsisent state
+        C.a = X()
+        C.a = X()
+
 
 if __name__ == '__main__':
     unittest.main()